Solid oxide cells (SOCs) generally include cells designed for different applications, such as solid oxide fuel cells (SOFCs) or solid oxide electrolysis cells (SOECs). Due to their common basic structure, the same cell may, for example, be used in SOFC applications as well as SOEC applications. Since in SOFCs fuel is fed into the cell and converted into power, while in SOECs power is applied to produce fuel, these cells are often referred to as ‘reversible’ SOCs.
Solid oxide cells may have various designs. Typical configurations include an electrolyte layer being sandwiched between two electrodes. During operation of the cell, usually at temperatures of about 500° C. to about 1100° C., one electrode is in contact with oxygen or air, while the other electrode is in contact with a fuel gas.
The most common manufacture processes suggested in the prior art comprise the manufacture of single cells. Generally, a support is provided, on which an electrode layer is formed, followed by the application of an electrolyte layer. The so formed half cell is dried and afterwards sintered, in some cases in a reducing atmosphere. Finally, a second electrode layer is formed thereon so as to obtain a complete cell. Alternatively, one of the electrode layers or the electrolyte layer may be used as a support layer, having a thickness of about 300 μm or more.
Under typical operating conditions, a single cell voltage is around 1±0.5 volt. To obtain high voltage and power from the SOCs, it is therefore necessary to stack many cells together. The most common manufacturing method for SOC planar stacks comprises the manufacture of single cells. The cells are subsequently stacked together with interconnects, current collectors, contact layers and seals. After assembly, the stacks are consolidated/sealed by heat treatment under a vertical load to ensure sealing as well as electrical contact between the components. Gas tight sealings are virtually important for the performance, durability and safely operation of a fuel cell as well as an electrolyser stack.
Silica based glass is a suitable sealing material for SOCs since the physical and chemical properties of glass can be tailored within a wide range. Different glass and glass-ceramic compositions have been examined within the group of alkali silicate, alkali aluminosilicates, alkaline earth silicates, alkaline earth aluminoborosilicates, and borosilicate glasses. However, even though promising results have been reported, none of them have been able to fulfill all the requirements of mechanical performance, e.g. viscosity and match of thermal expansion and chemical compatibility, e.g. wetting and bonding, at the same time.
In order to tailor the properties of the polymeric silica glass, network modifiers and network formers are added to the glass structure during melting. For example, Na containing compounds are added to increase the thermal expansion coefficient and Al containing compounds are added to balance the charge and thereby to avoid the bond breaking action of the Na, i.e. to prevent a depolymerisation of the SiO44− tetrahedron in the glass network.
However, especially when a SOC is used as an electrolysis cell, the conditions at the fuel gas electrode during operation of the cell are critical as water vapor has to be present. The high vapor pressure of the steam and the elevated temperatures result in corrosion of the glass seal. Some components in the glass seal, for example SiO2 and Na2O, may react with water and hydrogen so as to form volatile species with a high vapor pressure. Said species may then be transported into the adjacent electrode layers of the cell and are deposited at the reactive sites, thereby blocking and passivating these sites. Thus, the performance of the electrode decreases over time due to the decrease of active sites.